That is one of the things we have been considering. One of the problems - and it really only affects Speakers Corner, is that when discussions get heated we wanted to avoid the possibility that someone could edit their own post in the light of what another person subsequently said.
These discussions often move along more or less in real time, and post editing could be an issue in those circumstances.
Nevertheless, it's something that is still on our 'for consideration' list.
I've said it before, but a really good way of working in the forum is to write your posts into a word processor first, so it can be spell-checked, and then paste the text into the forum. Alternatively, use a browser that has a spell checker dictionary installed.